1. Field of the Invention
The present invention is related generally to floating platforms for tending marine operations, and in particular to a column stabilized, self-propelled semi-submersible vessel for servicing offshore petroleum production and drilling operations in severe ocean environments.
2. Description of the Prior Art
As energy demands increase, so does the need to explore and produce petroleum from offshore areas in deep and rough waters. Of vital concern to such offshore petroleum production operations is the need for adequate measures to be taken to minimize the effects of blowouts, fire and spills. There is a continuing interest in the advancement of concepts and equipment to adequately handle such offshore disasters in the rougher and deeper waters of the world.
With the development of major oil and gas offshore production facilities in new areas such as the North Sea, attention has been focused on the rough weather performance of existing heavy duty offshore work vessels. Weather conditions which are characteristic of the North Sea require a service vessel to exhibit sustained speed in a seaway, maneuverability, and stable sea keeping ability. For a work vessel operating in open water, seakeeping ability is a prime requirement, yet when the sea state rises over a mild chop, most conventional offshore work vessels are unable to maintain speed without severe pounding, pitching and rolling, which damages both cargo and vessel and makes travel uncomfortable for those aboard.
On location, stability is essential, because it is often difficult if not impossible to safely unload cargo from a rolling, pitching supply vessel onto a drilling or production platform. Experience has shown that considerable time is frequently lost, at a very high cost per hour, in delivering supplies and equipment to offshore platforms while waiting for weather conditions to improve. It is not unusual, for example, from North Sea supply and support operations to be curtailed as much as 25 percent of the time, due to adverse sea conditions. Also, in servicing underwater construction and salvage operations, a serious operational problem results from operating a diving bell from an unstable platform. When the diving bell rolls during launch and recovery, it causes difficulties to the diving personnel in the diving capsule. For this reason diving operations from standard supply and support vessels have been restricted to sea conditions no worse than sea state 5.
With the increased exploration and production activity, the construction of fixed production facilities has increased towards deeper waters. In the North Sea, for example, many major producers have installed platforms in over 200 to 400 feet of water. There is an increasing concern about the ability of the producers to cope with and provide services for disasters which occur in such deep and rough water environments. Although effective steps have been taken to prevent offshore blowouts and fires, there still exists the possibility of a disaster occurring in relatively deep waters.
According to a conventional procedure for coping with such deep water disasters, such as an offshore fire, a "work platform" is supported on the ocean floor at a fixed elevation adjacent to a drilling platform on which a blowout has occurred or which is on fire. The work platform provides a deck area from which debris can be cut away, the fire extinguished, and the well head shut off and capped. The practice of setting up such a work platform next to a burning production platform in such deep and rough water is clearly impractical from the standpoint of the time and expense involved. Studies which have compared operations in relatively calm waters such as those of the Gulf of Mexico and with relatively deep rough waters such as the North Sea have found that seas of six feet or higher occur less than five percent of the time in the Gulf, while in the North Sea, waves of these heights occur more than thirty five percent of the time generally and in some areas more than seventy percent of the time. Conventional barge equipment used to fight a fire and install a work platform cannot operate in seas greater than six feet. Therefore there is a serious and urgent need for a service vessel which can operate effectively in relatively rough seas.